Bulletin of the American Physical Society
67th Annual Meeting of the APS Division of Fluid Dynamics
Volume 59, Number 20
Sunday–Tuesday, November 23–25, 2014; San Francisco, California
Session D26: Turbulent Boundary Layers II |
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Chair: Joseph Klewicki, University of New Hampshire Room: 2007 |
Sunday, November 23, 2014 2:15PM - 2:28PM |
D26.00001: Fluctuating wall shear stress and velocity measurements in a turbulent boundary layer Rommel Pabon, Lawrence Ukeiley, Casey Barnard, Mark Sheplak Knowledge of mean wall shear stress on a surface can shed light on important performance parameters, but the fluctuating shear, even in simple flows, has not been as easily measured, and can be of interest in fundamental boundary layer research. Experiments on a flat plate model were performed to investigate the relationship between the wall shear stress and large scale events in the turbulent boundary layer. A MEMS based differential capacitance shear stress system with 1mm $\times$ 1mm floating element which can measure the fluctuating and static components of shear simultaneously, coupled with a hot wire anemometer were used for characterizing the turbulent boundary layer. Velocity profiles and turbulence statistics approaching the wall characterized the two dimensionality of the flat plate, and a trailing edge flap was used to impose a zero pressure gradient. The mean streamwise velocity profile was scaled by the friction velocity using the measured shear stress and independently compared to classical fits. Correlations between the fluctuating shear and measured velocities were used to elucidate the large scale events and to compare with previous fluctuating shear measurements for validation. [Preview Abstract] |
Sunday, November 23, 2014 2:28PM - 2:41PM |
D26.00002: Floating element measurements of wall-shear stress exerted by high-Reynolds-number turbulent boundary layers Woutijn J. Baars, Krishna M. Talluru, Nick Hutchins, Ivan Marusic Indirect methods to obtain the wall-shear stress $\tau_w$, such as the Clauser chart fit, necessitate inherent assumptions of the boundary layer. Therefore, direct methods are preferred to measure $\tau_w$ and subsequently obtain the friction velocity $U_{\tau} = \sqrt{\tau_w/\rho}$. Floating elements are genuinely small to obtain local wall-shear stress measurements, but cope with low signal-to-noise ratios since the signal scales with the surface area $(\propto l^2)$, where $l$ is the characteristic length, and the error forces scale with $hl$; $h$ represents the misalignment of the edges. Therefore, the capacious High Reynolds Number Boundary Layer Wind Tunnel at Melbourne incorporates a large floating element of 3m $\times$ 1m over which the changes in boundary layer parameters are negligible, and hence, local measurements of $U_{\tau}$ are made with high accuracy. Smooth-wall results follow the $U_{\infty}/U_{\tau} = 1/\kappa \ln \left({\rm Re}_{\theta}\right) + C$ trend within $\pm 1\%$ ($\kappa = 0.380$ and $C = 3.7$) for typical test conditions ranging from ${\rm Re}_{\theta} = 15,000$ to $45,000$. Moreover, the device is used to measure $U_{\tau}$ corresponding to rough walls, and boundary layers that are perturbed by flush-mounted control devices within the element. [Preview Abstract] |
Sunday, November 23, 2014 2:41PM - 2:54PM |
D26.00003: Large-scale structures in high Reynolds number turbulent boundary layers Christian J. K\"ahler, Nicolas Buchmann, Sven Scharnowski Large-scale turbulent flow structures in a flat plate turbulent boundary layer with zero pressure gradient were investigated with PIV in a closed-loop transonic wind tunnel at Ma $=$ 0.5 - 0.8, Re$_{\mathrm{\tau }}$ $=$ 5,100 - 9,500. The primary aim of the measurements was to simultaneously reach large Reynolds numbers and a low boundary layer thickness to model width ratio. The high Reynolds number is required to generate large scale structures with sufficient amplitude. The low ratio between the boundary layer thickness and the width of the model is necessary to avoid side effects resulting from the side walls and the blockage of the flow. Spatial two-point correlation and conditional correlations are calculated to determine the size and orientation of the large-scale flow structures. The difference between the correlation and conditional correlation analysis indicates that various large-scale structures with typical topologies exist and interact in turbulent boundary layer flows. [Preview Abstract] |
Sunday, November 23, 2014 2:54PM - 3:07PM |
D26.00004: Spectral scaling in boundary layers and pipes at very high Reynolds numbers Margit Vallikivi, Bharath Ganapathisubramani, Alexander Smits One-dimensional energy spectra in flat plate zero pressure gradient boundary layers and pipe flows are examined over a wide range of Reynolds numbers ($2,600 \leq Re_\tau \leq 72,500$). The peaks associated with the large-scale motions and superstructures in boundary layers behave as they do in pipe flows, with some minor differences. The location of the outer spectral peak (OSP) displays only a weak dependence on Reynolds number, and it occurs at the same wall-normal distance where the variances establish a logarithmic behavior. The outer-scaled wavelength of the OSP appears to decrease with increasing Reynolds number implying that the superstructures represent the inertial range of motions rather than the large scales per se. The location of the OSP appears to mark the start of a plateau that is consistent with a $k_x^{-1}$ slope in the spectrum and the logarithmic variation in the variances. It does not require full similarity between outer and wall-normal scaling on the wavenumber. The extent of $k_x^{-1}$ region depends on the wavelength of the OSP, which appears to emerge as a true inertial scale only at Reynolds numbers typical of atmospheric surface layers. [Preview Abstract] |
Sunday, November 23, 2014 3:07PM - 3:20PM |
D26.00005: Hybrid PIV-Particle Tracking Technique Applied to High Reynolds Number Turbulent Boundary Layer Measurements Julio Soria, Callum Atkinson, Nicolas Buchmann Zero-pressure gradient turbulent boundary layer (ZPGTBL) experiments in both the LTRAC water tunnel (LTRAC-WT) at Monash University and the University of Melbourne HRNTBL wind tunnel were undertaken up to $Re_{\tau} = 20,000$. Both experiments represent the flow along the centreline of each test section in the streamwise - wall-normal plane. Two PCO Dimax cameras with $2008 \times 2008$ pixel arrays were used in conjunction with a high-repetition laser to measure the time-resolved 2C-2D velocity field in the LTRAC-WT. The HRNTBL wind tunnel experiments employed nine PCO pro.4000 cameras each with a $4008 \times 2672$ pixel array. Two double cavity pulsed Nd:YAG lasers were used to acquire single-exposed PIV images simultaneously on all cameras to yield high spatial resolution 2C-2D velocity fields over a large extend of the ZPGTBL. In both cases, the single-exposed PIV images were analysed using multigrid cross-correlation PIV analysis, which has been enhanced to include a hybrid velocimetry step in which the PIV velocity is used as an estimator to a subsequent particle tracking refinement step. Statistics of the ZPGTBL velocity field as well as the high spatial resolution instantaneous structure and spatio-temporal structure of the ZPGTBL will be presented. [Preview Abstract] |
Sunday, November 23, 2014 3:20PM - 3:33PM |
D26.00006: Resolved measurements of the near-wall coherent structures Gerrit E. Elsinga, Yoshifumi Jodai The 3D coherent structures in the near-wall region of a turbulent boundary layer have been measured by time-resolved tomographic PIV. The Reynolds number based on the friction velocity was 814. The measurement volume extended from the wall up to a y$+$ of 170, and it spanned 680x620 wall units in the streamwise and spanwise direction respectively. The spatial resolution was 16 wall units, which corresponds to 5-6 Kolmogorov length scales. This is considered sufficiently resolved as to infer the nature of the vortical structures, which is still the subject of some debate. Compared to earlier 3D experiments this new data offers much improved spatial resolution within a relatively large flow domain and allows to follow the structures as they develop in time. Visualizations of vortical motions reveal quasi-streamwise vortices near the low speed streaks consistent with some of the proposed models for the near wall region. However, we also find clear evidence of hairpins in this region. Moreover, a new hairpin is observed to develop upstream of one of the pre-existing hairpins creating what may be considered a hairpin packet. This suggests auto-generation mechanisms to be present in the fully turbulent boundary layer. [Preview Abstract] |
Sunday, November 23, 2014 3:33PM - 3:46PM |
D26.00007: Shear-layer Effect on Wall Pressure Statistics in Turbulent Channel Flow Masayuki Sano, Tatsuya Tsuneyoshi, Yoshinobu Yamamoto, Yoshiyuki Tsuji The coherent structures are studied near wall region in turbulent channel flow by means of Direct Numerical Simulation (DNS). We analyze the correlation between coherent structures and wall pressure with positive and negative high amplitude peaks. DNS Reynolds numbers based on the friction velocity and the channel half-width are from 150 to 2000. The probability density functions of pressure indicate that there are different coherent structures associated with positive and negative pressure region. In order to evaluate the influence of the coherent structures on wall pressure, we visualize the conditioned streamwise velocity and second invariant of the deformation tensor conditioned by wall pressure. The second invariant of the deformation tensor is called Q criterion, which represents the intensity of vortices. It is found that the high pressure region is related to shear-layer in the buffer region and the negative pressure region is generated by small-scale vortices. Previous studies reported only the low-Reynolds number case, but the same results are confirmed in relatively high Reynolds number. The shear layer at Re$\tau =$150 and 1000 have the same spanwise scales. The result shows that the spanwise meandering of the large scale structure is normalized by the inner scale. [Preview Abstract] |
Sunday, November 23, 2014 3:46PM - 3:59PM |
D26.00008: Large scale and small scale interactions of pressure fluctuation in turbulent boundary layers Yoshiyuki Tsuji, Yoshinobu Yamamoto We study the interaction between large and small scale motions from the point of pressure fluctuation. Using the small pressure probe, both the static pressure and wall pressure fluctuations were measured inside the zero-pressure gradient boundary layer at relatively high Reynolds numbers. How the large scales in outside affect the small scales near wall is analyzed by means of statistical method. The correlation between pressure and pressure gradient indicates that the small scales distribute uniformly across the boundary layer. High amplitude positive and negative wall pressure fluctuations are also analyzed and found that they are associated with coherent motions inside the boundary layer. Another interesting aspect is the amplitude modulations of pressure and we would like to comment this topic in the presentation. [Preview Abstract] |
Sunday, November 23, 2014 3:59PM - 4:12PM |
D26.00009: Statistical comparison of coherent structures in fully developed turbulent pipe flow with and without drag reduction Francesca Sogaro, Robert Poole, David Dennis High-speed stereoscopic particle image velocimetry has been performed in fully developed turbulent pipe flow at moderate Reynolds numbers with and without a drag-reducing additive (an aqueous solution of high molecular weight polyacrylamide). Three-dimensional large and very large-scale motions (LSM and VLSM) are extracted from the flow fields by a detection algorithm and the characteristics for each case are statistically compared. The results show that the three-dimensional extent of VLSMs in drag reduced (DR) flow appears to increase significantly compared to their Newtonian counterparts. A statistical increase in azimuthal extent of DR VLSM is observed by means of two-point spatial autocorrelation of the streamwise velocity fluctuation in the radial-azimuthal plane. Furthermore, a remarkable increase in length of these structures is observed by three-dimensional two-point spatial autocorrelation. These results are accompanied by an analysis of the swirling strength in the flow field that shows a significant reduction in strength and number of the vortices for the DR flow. The findings suggest that the damping of the small scales due to polymer addition results in the undisturbed development of longer flow structures. [Preview Abstract] |
Sunday, November 23, 2014 4:12PM - 4:25PM |
D26.00010: Experimental investigation of turbulent channel flow over a compliant wall using tomographic PIV and Mach-Zehnder interferometry Cao Zhang, Rinaldo Miorini, Joseph Katz The time-resolved 3D flow field and 2D distribution of wall-normal deformation in turbulent channel flow over a compliant surface are simultaneously measured by a combination of tomographic PIV (TPIV) and Mach-Zehnder Interferometry (MZI). The compliant wall is made of PDMS, and the friction Reynolds number is $2.3\times 10^{3}$. The mean velocity profile in the log layer is consistent with that of a channel flow over a smooth rigid wall. The flow resolution of the TPIV measurement is enhanced using single-pixel ensemble correlations to resolve the buffer layer. Extensive calibrations of the MZI system show a wall-normal resolution of deformation in the order of 10 nm. The power spectral density of the surface deformation indicates a wide range of the time-scales. The streamwise wavenumber-frequency spectrum displays two main features: (i) An inclined band corresponding to deformations advected with the flow at approximately 80{\%} of the freestream speed, i.e. the velocity in the log layer. Their amplitudes are in the submicron range. (ii) Non advected, low frequency (\textless 500 Hz) events that are larger than the field-of-view, and have much higher amplitudes, up to 100 $\mu $m. Ongoing analyses examine the deformation-velocity and deformation-pressure correlations to identify structures that influence the interactions with the compliant wall. [Preview Abstract] |
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