Bulletin of the American Physical Society, Nov 19, 2017
various radii were performed to study influence of corner geometry on secondary flows. Unexpected... more various radii were performed to study influence of corner geometry on secondary flows. Unexpectedly, increased rounding of corners in rectangular ducts does not lead to monotonic trend towards pipe case. Instead, secondary vortices relocate close to regions of wall-curvature change. This behavior is connected to inhomogeneous interaction between near-wall bursting events, which are further characterized in this work with definition of their local preferential direction, and vorticity fluxes. Although these motions are relatively weak compared to streamwise velocity their effect on turbulence statistics and shear-stress distribution is very important and has not been sufficiently documented or fully understood. Flow through spanwiseperiodic channels, with sinusoidal function to define the geometry of wall, y w = ± h + A cos(ωz), was also studied as model flow that is parametrically changed using A and ω, while taking advantage of many resulting symmetries. Consequences on experimental facilities and comparisons between experiments and various numerical and theoretical models are discussed revealing the uniqueness of pipe flow.
Bulletin of the American Physical Society, Nov 26, 2013
Submitted for the DFD13 Meeting of The American Physical Society Latest Developments on Obtaining... more Submitted for the DFD13 Meeting of The American Physical Society Latest Developments on Obtaining Accurate Measurements with Pitot Tubes in ZPG Turbulent Boundary Layers HASSAN NAGIB, RI-CARDO VINUESA, IIT, Chicago -Ability of available Pitot tube corrections to provide accurate mean velocity profiles in ZPG boundary layers is re-examined following the recent work by Bailey et al. Measurements by Bailey et al., carried out with probes of diameters ranging from 0.2 to 1.89 mm, together with new data taken with larger diameters up to 12.82 mm, show deviations with respect to available high-quality datasets and hot-wire measurements in the same Reynolds number range. These deviations are significant in the buffer region around y + = 30 -40, and lead to disagreement in the von Kármán coefficient κ extracted from profiles. New forms for shear, near-wall and turbulence corrections are proposed, highlighting the importance of the latest one. Improved agreement in mean velocity profiles is obtained with new forms, where shear and near-wall corrections contribute with around 85%, and remaining 15% of the total correction comes from turbulence correction. Finally, available algorithms to correct wall position in profile measurements of wall-bounded flows are tested, using as benchmark the corrected Pitot measurements with artificially simulated probe shifts and blockage effects. We develop a new scheme, κB-Musker, which is able to accurately locate wall position.
Bulletin of the American Physical Society, Nov 20, 2006
The American Physical Society Spectral Characteristics of High Re Zero Pressure Gradient Turbulen... more The American Physical Society Spectral Characteristics of High Re Zero Pressure Gradient Turbulent Boundary Layers HASSAN NAGIB, KAPIL CHAUHAN, Illinois Institute of Technology, MICHAEL HITES, New Mexico State University, Las Cruces -One-dimensional spectra from data over the Re θ range between 4,100 and 23,700 in a zero pressure gradient turbulent boundary layer are studied. Detailed spectra computed from long time series of the streamwise velocity reveal a bi-modal distribution of energy in the buffer layer close to the wall. This confirms that the large scale, low frequency, motions of the boundary layer are important in the dynamics of the near wall region. The spectra exhibit distinct trends in the inner, outer and overlap region of the boundary layer, over limits that are consistent with those found from the mean velocity profile. Near the wall, the high frequencies/small scales are characterized by the length ν/u τ and velocity u τ , reflecting the classical inner scales of mean velocity. Away from the wall, low frequencies/large scales spectra scale with the length scale δ and velocity scale u τ , which are again consistent with the classical outer scales of mean velocity. Based on the inner and outer scalings, a spectral model is developed, which exhibits all the characteristics found in experimental measurements across the boundary layer. Results for Reynolds number trends of k -1 law, filtered time series and burst frequency will also be presented.
Bulletin of the American Physical Society, Nov 24, 2019
Revealing Some Roots to Our Uncertainty Regarding Values of Von Kármán Constants HASSAN NAGIB, II... more Revealing Some Roots to Our Uncertainty Regarding Values of Von Kármán Constants HASSAN NAGIB, IIT, Chicago,USA, LUCIA MAS-COTELLI, GABRIELE BELLANI, ALESSANDO TALAMELLI, Università di Bologna, Forlì, Italy -Three different fits of logarithmic dependence of centerline velocity, U + CL , in pipe flow at CICLoPE, with nearly equal representation of experimental data over range 8, 000 < Re τ < 40, 000 have been used to generate synthetic data that are densely spaced with Reynolds number. A higher order term proportional to inverse of Re τ was incorporated into one. Two approaches to uncertainty analysis of three sets of synthetic data were used to study dependence of uncertainty of extracted values of κ CL : Random Sampling of Full Range of Uncertainty and All Possible Permutations of Extreme Uncertainties. Role of low Reynolds number data and accuracy of pressure transducer used to measure pressure gradient along pipe were examined. Results reveal following mean values and uncertainties from measurements by the multiple-transducer pressure scanner that was used: κ CL = 0.44 ± 0.062 for all Reynolds numbers, and κ CL = 0.45 ± 0.038 for data with Re τ > 12, 000. More significantly, we conclude that CICLoPE requires a more accurate pressure scanning method for determining dp/dx; e.g., using a Scanivalve connected to a single more accurate pressure transducer. Such an approach has potential of reducing uncertainty by an order of magnitude.
Bulletin of the American Physical Society, Nov 20, 2005
The American Physical Society AISCOS a New Approach to Active Flow Control of Separation. P. REIN... more The American Physical Society AISCOS a New Approach to Active Flow Control of Separation. P. REINHARD, B. DEMANETT, J. KIEDAISCH, H. NAGIB, IIT, USA -Most studies of Active Flow Control (AFC) of separated flows have centered upon steady or periodic blowing, and Zero-Mass Flux (ZMF). However, ZMF with superposition of steady suction, or oscillatory suction, has recently demonstrated improved effectiveness in controlling large separated zones. Coupled with demonstrated hysteresis in the reattachment of such zones, we were led to the development of the concept of Adaptive Intermittent Suction Control of Separation (AISCOS). The concept and its effectiveness were demonstrated using a generic separated flow field over a hump model. While we have not tested AISCOS within a feedback arrangement to demonstrate its full potential, parametric testing of variables such as frequency and duty cycle have been carried out. When comparing the improvement as a result of AISCOS to other approaches such as ZMF, and to best potentially available improvement in peak C p value over the hump for a fully attached flow, we find that nearly double the change in peak C p could be hypothetically achieved in the case of ZMF. The best steady suction performance resulted in 22% of additional improvement over ZMF, and comparable improvements are achieved with AISCOS using various duty cycles. Most probably, the less than optimum location of the AFC slot on this model limited the potential of the improvement from the application of suction-based methods; the model and its AFC slot were originally designed to work most effectively with ZMF only.
Bulletin of the American Physical Society, Nov 24, 2009
Mean Flow and Wall Shear Measurements in Turbulent Boundary Layers with Various Imposed Complex P... more Mean Flow and Wall Shear Measurements in Turbulent Boundary Layers with Various Imposed Complex Pressure Gradients PAUL ROZIER, RICHARD DUNCAN, HASSAN NAGIB, IIT, Chicago -Mean flow velocity profiles with miniature Pitot probes, and wall-shear stress measurements with oil film interferometry, are used to document the development of high Reynolds number turbulent boundary layers on a flat plate under the influence of four different complex pressure gradients. The National Diagnostic Facility (NDF) is uniquely suited for establishing and documenting such flow conditions to serve as grounds for testing various turbulence models in well-known, two-dimensional boundary layers. Each of the four conditions starts with a zero pressure gradient and returns to it again along the test section. Two of the conditions include an adverse gradient region followed by a favorable one, and the other two cases experience the same two effects in the opposite sequence but to comparable magnitudes of free-stream velocity variations. The mean velocity profiles are measured and used to determine parameters such as the shape factor, the logarithmic overlap-region parameters, and the wake or outer flow parameters. The effect of the initial boundary layer conditions on the development of such complex flows is also examined by changing the position of the pressure gradient "hump" between two sets of two opposite gradients conditions. The results will be briefly compared to computations at IIT of nearly the same flow conditions.
APS Division of Fluid Dynamics Meeting Abstracts, Nov 1, 1998
Recently, several researchers including George and Castillo (1997), Barenblatt et al. (1997), and... more Recently, several researchers including George and Castillo (1997), Barenblatt et al. (1997), and Zagorola and Smits (1998), among others, have scrutinized the scaling of the mean velocity profile in turbulent pipe flow and turbulent boundary layers (TBLs). Much of the controversy stems from deliberations over whether a logarithmic law or a power law describes the mean velocity profiles in the
APS Division of Fluid Dynamics Meeting Abstracts, Nov 1, 2002
The mean velocity distribution in the overlap region of zero-pressure gradient boundary layers, o... more The mean velocity distribution in the overlap region of zero-pressure gradient boundary layers, over the range of Reynolds numbers 10,000 &lt; ReTheta &lt; 70,000, continues to be very accurately described by the Reynolds-number-independent log law. The streamwise normal-stress distribution will also be presented. The parameters of the logarithmic overlap region developed from these higher Re boundary layers continues to be
... Ahmed Hassan¶ The Boeing Company, Mesa, AZ ... Hassan, A. and Munts E. Transverse and Near T... more ... Ahmed Hassan¶ The Boeing Company, Mesa, AZ ... Hassan, A. and Munts E. Transverse and Near Tangent Synthetic Jets for Aerodynamic Flow Control, AIAA Paper ... Flux Pulsed Blowing, AIAA Paper 2001-0734, 39th Aerospace Sciences Meeting and Exhibit, Reno, NV, Jan. ...
This study is concerned primarily with the complex nature of leading edge flow separation occurri... more This study is concerned primarily with the complex nature of leading edge flow separation occurring on airfoils oscillating in a uniform flow field at low Reynolds numbers. The flow field past an oscillating airfoil and a fixed airfoil with an oscillating flap were investigated using various visualization techniques in water. The role of mean flow velocity, instan taneous angle of attack, mean angle of attack and amplitude and frequency of oscillation as well as the location of the support point are examined. The results which were obtained over a range of parameters substantially beyond previous studies include new information regarding the effect of these parameters on the nature and onset of separation. Two basic forms of leading edge separation have been observed. At low values of reduced fre-* quency ( u ) > 0.5) the separation resembles leading edge separation on stationary airfoils with the separated flow remaining detached from the upper * surface. At higher values, beyond mcr> a strong vortex (roller) is formed at the leading edge with the flow reattaching downstream from it. In addition to these two flow regimes, a third regime made its appearance at extremely high values of frequency and amplitude where the flow around the airfoil has collapsed. The frequency of oscillation is found to govern the angle, a , cr * at which leading edge separation occurs at low reduced frequencies (m < 0.2); 2 2 at higher frequencies acr occurs at the portion of the cycle where d a/dt is near its maximum.
SECURITY CLAISIFICETISh up 'THIS PAGE (II ~,sn D. . . Znt. rs ~B ~~Dr ~~? r ~l ~ra I I J I A?Iflk... more SECURITY CLAISIFICETISh up 'THIS PAGE (II ~,sn D. . . Znt. rs ~B ~~Dr ~~? r ~l ~ra I I J I A?IflkI D A( ~~ READ !NSTRUCT!ONS " ~~~~~' " " U ~~~~~~J IW ~~~I ~ I ~~~J I ~Ji ~ ~~~~~~ BEFORE COMPLETIN G FORM I ~~REPORT NUNIER 2. 30VT ACCESSION NO 3. REd T 5 CATALOG MuMUR
Using a special calibration tunnel developed during the course of this study, the static and dyna... more Using a special calibration tunnel developed during the course of this study, the static and dynamic response of several kinds of commercially available hot-film probes with single and multiple sensors of the cylindrical-fiber type are examined.
The methodology of matched asymptotic expansions or &quot;generalized boundary layer theory&a... more The methodology of matched asymptotic expansions or &quot;generalized boundary layer theory&quot; for large Reynolds number ReW (based on friction velocity and outer length scale), pioneered by Prandtl (1), is used to extract from measured or computed mean velocity profiles in turbulent channel and pipe flows their limiting behavior at infinite ReW . After fitting an &quot;outer expansion&quot; in terms of suitable functions of the outer wall-normal coordinateKto the data, the construction of composite expansions is used &quot;in reverse&quot; to extract the &quot;inner expansion&quot;. Its leading term of order O(ReW 0 ) represents the near-wall solution for infinite ReW which is found to be identical for channels and pipes. For large values of the inner wall- normal coordinate y+ , this limiting inner expansion for the streamwise velocity is furthermore shown to be well described by Prandtl&#x27;s famous &quot;log-law&quot;.
Velocity and wall shear stress were measured using hot-wire anemometry in two distinct turbulent ... more Velocity and wall shear stress were measured using hot-wire anemometry in two distinct turbulent boundary layers in the National Diagnostic Facility and have demonstrated some new and unique features of the high Reynolds number turbulent boundary layer. In agreement with the literature, mean velocity profiles showed that the socalled logarithmic region grew continuously for increasing Reynolds number for all of the boundary layers confirming an extended disparity between small and large scale motion. It was also observed from the mean velocity profiles that surface roughness elements of less than 2 viscous lengths caused a noticeable overshoot of the log-law line as the Reynolds number increased which indicated the importance of surface roughness at high Reynolds number even when the roughness height was within the viscous sublayer. Spectra computed from long time-series of the streamwise velocity revealed a bi-modal distribution of energy in the boundary layer close to the wall (50 viscous units) asserting that the large-scale, lowfrequency motions of the boundary layer are important in the near-wall region. Direct shear stress measurements using wire-on-wall and MEMS sensors were compared to the Clauser prediction, and it was seen that the Clauser scaling tended to mask the streamwise (Re.,) dependencies of the boundary layer quantities.
Journal of Wind Engineering and Industrial Aerodynamics, Oct 1, 1979
A family of thick turbulent boundary layers with different characteristics was generated by the c... more A family of thick turbulent boundary layers with different characteristics was generated by the counter-jet technique and used to simulate neutral atmospheric surface layers. These selected "test boundary layers" were utilized to examine the response of the flowfield near a building model to variations in the characteristics of these turbulent layers. The counter-jets, applied at the wall near the entrance of a moderate length test section, permit rapid alteration of reproducible boundary layers from outside the tunnel while experiments are in progress. The mean and unsteady characteristics of the test boundary layers were documented through prof'fles of mean velocity, turbulence intensity, integral scales, and Reynolds stress, and through velocity spectra. The substantial deviations of the atmospheric surface layer, in time and space, from over-idealized standard layers led us to the "sensitivity testing" approach. For example, using simulated surface layers which bracket the variations in the atmospheric conditions, this approach pxovides valuable information through the bracketing of wind loads or pressure distributions by extreme values. This ~sensitivity study employs four substantially different test boundary layers, and involves a building model at two orientations to the wind. Changes in salient features of the flow over and around the bluff body and in its wake were revealed through profiles of mean velocity and turbulence intensity. In particular, the development of the shear layers above and downstream of the building displayed a strong dependence on the test boundary layer used.
Direct numerical simulations (DNSs) are performed to analyse the secondary flow of Prandtl’s seco... more Direct numerical simulations (DNSs) are performed to analyse the secondary flow of Prandtl’s second kind in fully developed spanwise-periodic channels with in-plane sinusoidal walls. The secondary flow is characterized for different combinations of wave parameters defining the wall geometry at $Re_{h}=2500$ and 5000, where $h$ is the half-height of the channel. The total cross-flow rate in the channel $Q_{yz}$ is defined along with a theoretical model to predict its behaviour. Interaction between the secondary flows from opposite walls is observed if $\unicode[STIX]{x1D706}\simeq h\simeq A$, where $A$ and $\unicode[STIX]{x1D706}$ are the amplitude and wavelength of the sinusoidal function defining the wall geometry. As the outer-scaled wavelength ($\unicode[STIX]{x1D706}/h$) is reduced, the secondary vortices become smaller and faster, increasing the total cross-flow rate per wall. However, if the inner-scaled wavelength ($\unicode[STIX]{x1D706}^{+}$) is below 130 viscous units, the cross-flow decays for smaller wavelengths. By analysing cases in which the wavelength of the wall is much smaller than the half-height of the channel $\unicode[STIX]{x1D706}\ll h$, we show that the cross-flow distribution depends almost entirely on the separation between the scales of the instantaneous vortices, where the upper and lower bounds are determined by $\unicode[STIX]{x1D706}/h$ and $\unicode[STIX]{x1D706}^{+}$, respectively. Therefore, the distribution of the secondary flow relative to the size of the wave at a given $Re_{h}$ can be replicated at higher $Re_{h}$ by decreasing $\unicode[STIX]{x1D706}/h$ and keeping $\unicode[STIX]{x1D706}^{+}$ constant. The mechanisms that contribute to the mean cross-flow are analysed in terms of the Reynolds stresses and using quadrant analysis to evaluate the probability density function of the bursting events. These events are further classified with respect to the sign of their instantaneous spanwise velocities. Sweeping events and ejections are preferentially located in the valleys and peaks of the wall, respectively. The sweeps direct the instantaneous cross-flow from the core of the channel towards the wall, turning in the wall-tangent direction towards the peaks. The ejections drive the instantaneous cross-flow from the near-wall region towards the core. This preferential behaviour is identified as one of the main contributors to the secondary flow.
Bulletin of the American Physical Society, Nov 19, 2017
various radii were performed to study influence of corner geometry on secondary flows. Unexpected... more various radii were performed to study influence of corner geometry on secondary flows. Unexpectedly, increased rounding of corners in rectangular ducts does not lead to monotonic trend towards pipe case. Instead, secondary vortices relocate close to regions of wall-curvature change. This behavior is connected to inhomogeneous interaction between near-wall bursting events, which are further characterized in this work with definition of their local preferential direction, and vorticity fluxes. Although these motions are relatively weak compared to streamwise velocity their effect on turbulence statistics and shear-stress distribution is very important and has not been sufficiently documented or fully understood. Flow through spanwiseperiodic channels, with sinusoidal function to define the geometry of wall, y w = ± h + A cos(ωz), was also studied as model flow that is parametrically changed using A and ω, while taking advantage of many resulting symmetries. Consequences on experimental facilities and comparisons between experiments and various numerical and theoretical models are discussed revealing the uniqueness of pipe flow.
Bulletin of the American Physical Society, Nov 26, 2013
Submitted for the DFD13 Meeting of The American Physical Society Latest Developments on Obtaining... more Submitted for the DFD13 Meeting of The American Physical Society Latest Developments on Obtaining Accurate Measurements with Pitot Tubes in ZPG Turbulent Boundary Layers HASSAN NAGIB, RI-CARDO VINUESA, IIT, Chicago -Ability of available Pitot tube corrections to provide accurate mean velocity profiles in ZPG boundary layers is re-examined following the recent work by Bailey et al. Measurements by Bailey et al., carried out with probes of diameters ranging from 0.2 to 1.89 mm, together with new data taken with larger diameters up to 12.82 mm, show deviations with respect to available high-quality datasets and hot-wire measurements in the same Reynolds number range. These deviations are significant in the buffer region around y + = 30 -40, and lead to disagreement in the von Kármán coefficient κ extracted from profiles. New forms for shear, near-wall and turbulence corrections are proposed, highlighting the importance of the latest one. Improved agreement in mean velocity profiles is obtained with new forms, where shear and near-wall corrections contribute with around 85%, and remaining 15% of the total correction comes from turbulence correction. Finally, available algorithms to correct wall position in profile measurements of wall-bounded flows are tested, using as benchmark the corrected Pitot measurements with artificially simulated probe shifts and blockage effects. We develop a new scheme, κB-Musker, which is able to accurately locate wall position.
Bulletin of the American Physical Society, Nov 20, 2006
The American Physical Society Spectral Characteristics of High Re Zero Pressure Gradient Turbulen... more The American Physical Society Spectral Characteristics of High Re Zero Pressure Gradient Turbulent Boundary Layers HASSAN NAGIB, KAPIL CHAUHAN, Illinois Institute of Technology, MICHAEL HITES, New Mexico State University, Las Cruces -One-dimensional spectra from data over the Re θ range between 4,100 and 23,700 in a zero pressure gradient turbulent boundary layer are studied. Detailed spectra computed from long time series of the streamwise velocity reveal a bi-modal distribution of energy in the buffer layer close to the wall. This confirms that the large scale, low frequency, motions of the boundary layer are important in the dynamics of the near wall region. The spectra exhibit distinct trends in the inner, outer and overlap region of the boundary layer, over limits that are consistent with those found from the mean velocity profile. Near the wall, the high frequencies/small scales are characterized by the length ν/u τ and velocity u τ , reflecting the classical inner scales of mean velocity. Away from the wall, low frequencies/large scales spectra scale with the length scale δ and velocity scale u τ , which are again consistent with the classical outer scales of mean velocity. Based on the inner and outer scalings, a spectral model is developed, which exhibits all the characteristics found in experimental measurements across the boundary layer. Results for Reynolds number trends of k -1 law, filtered time series and burst frequency will also be presented.
Bulletin of the American Physical Society, Nov 24, 2019
Revealing Some Roots to Our Uncertainty Regarding Values of Von Kármán Constants HASSAN NAGIB, II... more Revealing Some Roots to Our Uncertainty Regarding Values of Von Kármán Constants HASSAN NAGIB, IIT, Chicago,USA, LUCIA MAS-COTELLI, GABRIELE BELLANI, ALESSANDO TALAMELLI, Università di Bologna, Forlì, Italy -Three different fits of logarithmic dependence of centerline velocity, U + CL , in pipe flow at CICLoPE, with nearly equal representation of experimental data over range 8, 000 < Re τ < 40, 000 have been used to generate synthetic data that are densely spaced with Reynolds number. A higher order term proportional to inverse of Re τ was incorporated into one. Two approaches to uncertainty analysis of three sets of synthetic data were used to study dependence of uncertainty of extracted values of κ CL : Random Sampling of Full Range of Uncertainty and All Possible Permutations of Extreme Uncertainties. Role of low Reynolds number data and accuracy of pressure transducer used to measure pressure gradient along pipe were examined. Results reveal following mean values and uncertainties from measurements by the multiple-transducer pressure scanner that was used: κ CL = 0.44 ± 0.062 for all Reynolds numbers, and κ CL = 0.45 ± 0.038 for data with Re τ > 12, 000. More significantly, we conclude that CICLoPE requires a more accurate pressure scanning method for determining dp/dx; e.g., using a Scanivalve connected to a single more accurate pressure transducer. Such an approach has potential of reducing uncertainty by an order of magnitude.
Bulletin of the American Physical Society, Nov 20, 2005
The American Physical Society AISCOS a New Approach to Active Flow Control of Separation. P. REIN... more The American Physical Society AISCOS a New Approach to Active Flow Control of Separation. P. REINHARD, B. DEMANETT, J. KIEDAISCH, H. NAGIB, IIT, USA -Most studies of Active Flow Control (AFC) of separated flows have centered upon steady or periodic blowing, and Zero-Mass Flux (ZMF). However, ZMF with superposition of steady suction, or oscillatory suction, has recently demonstrated improved effectiveness in controlling large separated zones. Coupled with demonstrated hysteresis in the reattachment of such zones, we were led to the development of the concept of Adaptive Intermittent Suction Control of Separation (AISCOS). The concept and its effectiveness were demonstrated using a generic separated flow field over a hump model. While we have not tested AISCOS within a feedback arrangement to demonstrate its full potential, parametric testing of variables such as frequency and duty cycle have been carried out. When comparing the improvement as a result of AISCOS to other approaches such as ZMF, and to best potentially available improvement in peak C p value over the hump for a fully attached flow, we find that nearly double the change in peak C p could be hypothetically achieved in the case of ZMF. The best steady suction performance resulted in 22% of additional improvement over ZMF, and comparable improvements are achieved with AISCOS using various duty cycles. Most probably, the less than optimum location of the AFC slot on this model limited the potential of the improvement from the application of suction-based methods; the model and its AFC slot were originally designed to work most effectively with ZMF only.
Bulletin of the American Physical Society, Nov 24, 2009
Mean Flow and Wall Shear Measurements in Turbulent Boundary Layers with Various Imposed Complex P... more Mean Flow and Wall Shear Measurements in Turbulent Boundary Layers with Various Imposed Complex Pressure Gradients PAUL ROZIER, RICHARD DUNCAN, HASSAN NAGIB, IIT, Chicago -Mean flow velocity profiles with miniature Pitot probes, and wall-shear stress measurements with oil film interferometry, are used to document the development of high Reynolds number turbulent boundary layers on a flat plate under the influence of four different complex pressure gradients. The National Diagnostic Facility (NDF) is uniquely suited for establishing and documenting such flow conditions to serve as grounds for testing various turbulence models in well-known, two-dimensional boundary layers. Each of the four conditions starts with a zero pressure gradient and returns to it again along the test section. Two of the conditions include an adverse gradient region followed by a favorable one, and the other two cases experience the same two effects in the opposite sequence but to comparable magnitudes of free-stream velocity variations. The mean velocity profiles are measured and used to determine parameters such as the shape factor, the logarithmic overlap-region parameters, and the wake or outer flow parameters. The effect of the initial boundary layer conditions on the development of such complex flows is also examined by changing the position of the pressure gradient "hump" between two sets of two opposite gradients conditions. The results will be briefly compared to computations at IIT of nearly the same flow conditions.
APS Division of Fluid Dynamics Meeting Abstracts, Nov 1, 1998
Recently, several researchers including George and Castillo (1997), Barenblatt et al. (1997), and... more Recently, several researchers including George and Castillo (1997), Barenblatt et al. (1997), and Zagorola and Smits (1998), among others, have scrutinized the scaling of the mean velocity profile in turbulent pipe flow and turbulent boundary layers (TBLs). Much of the controversy stems from deliberations over whether a logarithmic law or a power law describes the mean velocity profiles in the
APS Division of Fluid Dynamics Meeting Abstracts, Nov 1, 2002
The mean velocity distribution in the overlap region of zero-pressure gradient boundary layers, o... more The mean velocity distribution in the overlap region of zero-pressure gradient boundary layers, over the range of Reynolds numbers 10,000 &lt; ReTheta &lt; 70,000, continues to be very accurately described by the Reynolds-number-independent log law. The streamwise normal-stress distribution will also be presented. The parameters of the logarithmic overlap region developed from these higher Re boundary layers continues to be
... Ahmed Hassan¶ The Boeing Company, Mesa, AZ ... Hassan, A. and Munts E. Transverse and Near T... more ... Ahmed Hassan¶ The Boeing Company, Mesa, AZ ... Hassan, A. and Munts E. Transverse and Near Tangent Synthetic Jets for Aerodynamic Flow Control, AIAA Paper ... Flux Pulsed Blowing, AIAA Paper 2001-0734, 39th Aerospace Sciences Meeting and Exhibit, Reno, NV, Jan. ...
This study is concerned primarily with the complex nature of leading edge flow separation occurri... more This study is concerned primarily with the complex nature of leading edge flow separation occurring on airfoils oscillating in a uniform flow field at low Reynolds numbers. The flow field past an oscillating airfoil and a fixed airfoil with an oscillating flap were investigated using various visualization techniques in water. The role of mean flow velocity, instan taneous angle of attack, mean angle of attack and amplitude and frequency of oscillation as well as the location of the support point are examined. The results which were obtained over a range of parameters substantially beyond previous studies include new information regarding the effect of these parameters on the nature and onset of separation. Two basic forms of leading edge separation have been observed. At low values of reduced fre-* quency ( u ) > 0.5) the separation resembles leading edge separation on stationary airfoils with the separated flow remaining detached from the upper * surface. At higher values, beyond mcr> a strong vortex (roller) is formed at the leading edge with the flow reattaching downstream from it. In addition to these two flow regimes, a third regime made its appearance at extremely high values of frequency and amplitude where the flow around the airfoil has collapsed. The frequency of oscillation is found to govern the angle, a , cr * at which leading edge separation occurs at low reduced frequencies (m < 0.2); 2 2 at higher frequencies acr occurs at the portion of the cycle where d a/dt is near its maximum.
SECURITY CLAISIFICETISh up 'THIS PAGE (II ~,sn D. . . Znt. rs ~B ~~Dr ~~? r ~l ~ra I I J I A?Iflk... more SECURITY CLAISIFICETISh up 'THIS PAGE (II ~,sn D. . . Znt. rs ~B ~~Dr ~~? r ~l ~ra I I J I A?IflkI D A( ~~ READ !NSTRUCT!ONS " ~~~~~' " " U ~~~~~~J IW ~~~I ~ I ~~~J I ~Ji ~ ~~~~~~ BEFORE COMPLETIN G FORM I ~~REPORT NUNIER 2. 30VT ACCESSION NO 3. REd T 5 CATALOG MuMUR
Using a special calibration tunnel developed during the course of this study, the static and dyna... more Using a special calibration tunnel developed during the course of this study, the static and dynamic response of several kinds of commercially available hot-film probes with single and multiple sensors of the cylindrical-fiber type are examined.
The methodology of matched asymptotic expansions or &quot;generalized boundary layer theory&a... more The methodology of matched asymptotic expansions or &quot;generalized boundary layer theory&quot; for large Reynolds number ReW (based on friction velocity and outer length scale), pioneered by Prandtl (1), is used to extract from measured or computed mean velocity profiles in turbulent channel and pipe flows their limiting behavior at infinite ReW . After fitting an &quot;outer expansion&quot; in terms of suitable functions of the outer wall-normal coordinateKto the data, the construction of composite expansions is used &quot;in reverse&quot; to extract the &quot;inner expansion&quot;. Its leading term of order O(ReW 0 ) represents the near-wall solution for infinite ReW which is found to be identical for channels and pipes. For large values of the inner wall- normal coordinate y+ , this limiting inner expansion for the streamwise velocity is furthermore shown to be well described by Prandtl&#x27;s famous &quot;log-law&quot;.
Velocity and wall shear stress were measured using hot-wire anemometry in two distinct turbulent ... more Velocity and wall shear stress were measured using hot-wire anemometry in two distinct turbulent boundary layers in the National Diagnostic Facility and have demonstrated some new and unique features of the high Reynolds number turbulent boundary layer. In agreement with the literature, mean velocity profiles showed that the socalled logarithmic region grew continuously for increasing Reynolds number for all of the boundary layers confirming an extended disparity between small and large scale motion. It was also observed from the mean velocity profiles that surface roughness elements of less than 2 viscous lengths caused a noticeable overshoot of the log-law line as the Reynolds number increased which indicated the importance of surface roughness at high Reynolds number even when the roughness height was within the viscous sublayer. Spectra computed from long time-series of the streamwise velocity revealed a bi-modal distribution of energy in the boundary layer close to the wall (50 viscous units) asserting that the large-scale, lowfrequency motions of the boundary layer are important in the near-wall region. Direct shear stress measurements using wire-on-wall and MEMS sensors were compared to the Clauser prediction, and it was seen that the Clauser scaling tended to mask the streamwise (Re.,) dependencies of the boundary layer quantities.
Journal of Wind Engineering and Industrial Aerodynamics, Oct 1, 1979
A family of thick turbulent boundary layers with different characteristics was generated by the c... more A family of thick turbulent boundary layers with different characteristics was generated by the counter-jet technique and used to simulate neutral atmospheric surface layers. These selected "test boundary layers" were utilized to examine the response of the flowfield near a building model to variations in the characteristics of these turbulent layers. The counter-jets, applied at the wall near the entrance of a moderate length test section, permit rapid alteration of reproducible boundary layers from outside the tunnel while experiments are in progress. The mean and unsteady characteristics of the test boundary layers were documented through prof'fles of mean velocity, turbulence intensity, integral scales, and Reynolds stress, and through velocity spectra. The substantial deviations of the atmospheric surface layer, in time and space, from over-idealized standard layers led us to the "sensitivity testing" approach. For example, using simulated surface layers which bracket the variations in the atmospheric conditions, this approach pxovides valuable information through the bracketing of wind loads or pressure distributions by extreme values. This ~sensitivity study employs four substantially different test boundary layers, and involves a building model at two orientations to the wind. Changes in salient features of the flow over and around the bluff body and in its wake were revealed through profiles of mean velocity and turbulence intensity. In particular, the development of the shear layers above and downstream of the building displayed a strong dependence on the test boundary layer used.
Direct numerical simulations (DNSs) are performed to analyse the secondary flow of Prandtl’s seco... more Direct numerical simulations (DNSs) are performed to analyse the secondary flow of Prandtl’s second kind in fully developed spanwise-periodic channels with in-plane sinusoidal walls. The secondary flow is characterized for different combinations of wave parameters defining the wall geometry at $Re_{h}=2500$ and 5000, where $h$ is the half-height of the channel. The total cross-flow rate in the channel $Q_{yz}$ is defined along with a theoretical model to predict its behaviour. Interaction between the secondary flows from opposite walls is observed if $\unicode[STIX]{x1D706}\simeq h\simeq A$, where $A$ and $\unicode[STIX]{x1D706}$ are the amplitude and wavelength of the sinusoidal function defining the wall geometry. As the outer-scaled wavelength ($\unicode[STIX]{x1D706}/h$) is reduced, the secondary vortices become smaller and faster, increasing the total cross-flow rate per wall. However, if the inner-scaled wavelength ($\unicode[STIX]{x1D706}^{+}$) is below 130 viscous units, the cross-flow decays for smaller wavelengths. By analysing cases in which the wavelength of the wall is much smaller than the half-height of the channel $\unicode[STIX]{x1D706}\ll h$, we show that the cross-flow distribution depends almost entirely on the separation between the scales of the instantaneous vortices, where the upper and lower bounds are determined by $\unicode[STIX]{x1D706}/h$ and $\unicode[STIX]{x1D706}^{+}$, respectively. Therefore, the distribution of the secondary flow relative to the size of the wave at a given $Re_{h}$ can be replicated at higher $Re_{h}$ by decreasing $\unicode[STIX]{x1D706}/h$ and keeping $\unicode[STIX]{x1D706}^{+}$ constant. The mechanisms that contribute to the mean cross-flow are analysed in terms of the Reynolds stresses and using quadrant analysis to evaluate the probability density function of the bursting events. These events are further classified with respect to the sign of their instantaneous spanwise velocities. Sweeping events and ejections are preferentially located in the valleys and peaks of the wall, respectively. The sweeps direct the instantaneous cross-flow from the core of the channel towards the wall, turning in the wall-tangent direction towards the peaks. The ejections drive the instantaneous cross-flow from the near-wall region towards the core. This preferential behaviour is identified as one of the main contributors to the secondary flow.
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