Experimental analysis of the log-law at adverse pressure gradient

The experimental data for the mean velocity are analysed in the inner layer for a turbulent boundary layer at significant adverse pressure gradient and Reynolds numbers up to Re-theta=57000. The aim is to determine the resilience of the log law for the mean velocity, the possible change of the von Karman constant kappa and the appearance of a square-root law above the log law at significant adverse pressure gradients. In the wind-tunnel experiment, the adverse pressure gradient is imposed by an S-shaped deflection of the contour model which is mounted on a wind-tunnel sidewall. A large-scale particle imaging velocimetry method is applied to measure the streamwise evolution of the flow over a streamwise distance of 15 boundary layer thicknesses. In the adverse pressure gradient region, microscopic and three-dimensional Lagrangian particle tracking velocimetry are used to measure the mean velocity and the Reynolds stresses down to the viscous sublayer. Oil-film interferometry is used to determine the wall shear stress. The log law in the mean-velocity profile is found to be a robust feature at adverse pressure gradient, but its region is thinner than its zero pressure gradient counterpart, and its slope is altered. A square-root law emerges above the log law, extending to the wall distance the log law typically occupies at zero pressure gradient. Lower values for kappa are found than for zero pressure gradient turbulent boundary layers, but the reduction is within the uncertainty of the measurement

Modification of the SSG/LRR-omega RSM for adverse pressure gradients using turbulent boundary layer experiments at high Re

A modification of the SSG/LRR-omega model for turbulent boundary layers in adverse pressure gradient is presented. The modification is based on a new wall law for the mean velocity at adverse pressure gradient. The wall law is found from two new joint DLR/UniBw experiments and from the analysis of a data base from the literature. The mean velocity profile in the inner layer is found to consist of a log-law region, which is thinner than its zero pressure gradient counterpart, and a half-power law region above the log law. An empirical correlation for the wall-distance of the transition from the log-law to the half-power law is presented. Then a modification of the omega-equation to account for a half-power law behaviour of the mean velocity is described. The modified SSG/LRR-omega model is then applied to the two joint DLR/UniBw experiments. The modification leads to a reduction of the mean velocity in the inner part of the boundary layer and makes the model more susceptible for flow separation, which is in good agreement with the experimental data

A wall-law for adverse pressure gradient flows and modification for k-ω models

The reliable prediction of low-speed flow separation of a turbulent boundary layer (TBL) on a smooth surface due to an adverse pressure gradient (APG) using RANS-based CFD methods is still an open issue. In the literature, there is no law-of-the-wall for APG flows, which could be used for a modification of RANS models

Optical skin friction measurements in a turbulent boundary layer with pressure gradient

The present study is a part of a high-Reynolds-number experiment designed to establish a reliable data base helping to improve RANS turbulence models. In this work skin friction measurements were conducted in turbulent flows with significant pressure gradient using oil-film interferometry. The experiments were performed on a large-scale test model for three different flow velocities of 10, 23 and 36 m/s. Analysis of the skin friction distributions indicates that the state of the turbulent boundary layer remained out of equilibrium along the whole area investigated

Optische Messung der Wandschubspannung in turbulenten Grenzschichten mit Druckgradient

The present study is a part of a high-Reynolds-number experiment designed to establish a reliable data base helping to improve RANS turbulence models. In this work skin friction measurements were conducted in turbulent flows with significant pressure gradient using oil-film interferometry. The experiments were performed on a large-scale test model for three different flow velocities of 10, 23 and 36 m/s. Analysis of the skin friction distributions indicates that the state of the turbulent boundary layer remained out of equilibrium along the whole area investigated

Experimentelle Untersuchung von kohärenten Strukturen in turbulenten Grenzschichten

Eine Reduktion des Treibstoffverbrauchs und der Schadstoffemissionen von zivilen Flugzeugen ist eines der essentiellen Ziele der aktuellen Luftfahrt-Forschung. Entscheidend für die Steigerung der aerodynamischen Effizienz ist die Verringerung des Strömungswiderstandes, insbesondere des Reibungswiderstandes, welcher in der turbulenten Grenzschicht entsteht und etwa 55% des Gesamtwiderstandes moderner Verkehrflugzeuge ausmacht [1]. Grundlegende Forschungsarbeiten der letzten Jahre haben gezeigt, dass ein detailiiertes Verständnis von turbulenten Grenzschichten, insbesondere der Dynamik von großskaligen kohärenten Strukturen, neue Ansätze zur aktiven und passiven Strömungsbeeinflussung liefern kann

Lagrangian 3D particle tracking in high-speed flows: Shake-The-Box for multi-pulse systems

The Shake The Box (STB) particle tracking technique, recently introduced for time resolved 3D particle image velocimetry (PIV) images, is applied here to data from a multipulse investigation of a turbulent boundary layer flow with adverse pressure gradient in air at 36 m/s (Reτ = 10,650). The multipulse acquisition strategy allows for the recording of four pulse long timeresolved sequences with a time separation of a few microseconds. The experimental setup consists of a dualimaging System and a dualdoublecavity laser emitting orthogonal polarization directions to separate the four pulses. The STB particle triangulation and tracking strategy is adapted here to cope with the limited amount of realizations available along the time sequence and to take advantage of the ghost track reduction offered by the use of two independent imaging systems. Furthermore, a correction scheme to compensate for camera vibrations is discussed, together with a method to accurately identify the position of the wall within the measurement domain. Results show that approximately 80,000 tracks can be instantaneously reconstructed within the measurement volume, enabling the evaluation of both dense velocity fields, suitable for spatial gradients evaluation, and highly spatially resolved boundary layer profiles. Turbulent boundary layer profiles obtained from ensemble averaging of the STB tracks are compared to results from 2DPIV and longrange micro particle tracking velocimetry; the comparison shows the capability of the STB approach in delivering accurate results across a wide range of scales

Modification of the SSG/LRR-Omega Model for Turbulent Boundary Layer Flows in an Adverse Pressure Gradient

A modification of the RANS turbulence model SSG/LRR-omega for turbulent boundary layers in an adverse pressure gradient is presented. The modification is based on a wall law for the mean velocity, in which the log law is progressively eroded in an adverse pressure gradient and an extended wall law (designated loosely as a half-power law) emerges above the log law. An augmentation term for the half-power law region is derived from the analysis of the boundary-layer equation for the specific rate of dissipation omega. An extended data structure within the RANS solver provides, for each viscous wall point, the field points on a wall normal line. This enables the evaluation of characteristic boundary layer parameters for the local activation of the augmentation term. The modification is calibrated using a joint DLR/UniBw turbulent boundary layer experiment. The modified model yields an improved predictive accuracy for flow separation. Finally, the applicability of the modified model to a 3D wing-body configuration is demonstrated

A new experiment of a turbulent boundary layer flow at adverse pressure gradient for validation and improvement of RANS turbulence models

Abstract/Kurzfassung: We present a turbulent boundary layer flow experiment at a significant adverse pressure gradient and at a high Reynolds number. We describe the design of the test case and the set-up in the wind-tunnel so that the flow is suitable for the validation of RANS turbulence models. We present RANS simulations using the SST k-w model, and the SSG/LRR-w and the JHh-v2 Reynolds stress model. We show that the predictive accuraccy in the adverse pressure gradient region is significantly effected by the predictive accuracy in the upstream located region of a favourable pressure gradient, where the mean flow follows a curved surface. The effects of flow history have to be taken into account when assessing turbulence models in the adverse pressure gradient region. We study in detail the role of the cross-diffusion term in the w-equation for favourable and adverse pressure gradient