Artur Dróżdż

Detection of coherent structures in a turbulent boundary layer with zero, favourable and adverse pressure gradients

The paper presents the analysis of turbulent coherent structures found in a turbulent boundary layer subjected to zero, favourable and adverse pressure gradient. The analysis of the shape of conditionally averaged traces of u and v velocity components obtained by VITA (Variable Interval Time Averaging) technique based on signals recorded by X-wire probe allows to detect four types of coherent structures. The paper documents a presence of coherent vortical structures of positive or negative vorticity moving upward and downward. It is shown also that the acceleration and deceleration of the mean streamwise velocity, due to the pressure gradient, modifies the vortex convection velocity magnitude and the angle. This effect is clearly seen especially for the adverse pressure gradient flow, where Q events in first and fourth quadrants are enhanced near the wall.

Influence of pressure gradient on streamwise skewness factor in turbulent boundary layer

The paper shows an effect of favourable and adverse pressure gradients on turbulent boundary layer. The skewness factor of streamwise velocity component was chosen as a measure of the pressure gradient impact. It appears that skewness factor is an indicator of convection velocity of coherent structures, which is not always equal to the average flow velocity. The analysis has been performed based upon velocity profiles measured with hot-wire technique in turbulent boundary layer with pressure gradient corresponding to turbomachinery conditions. The results show that the skewness factor decreases in the flow region subjected to FPG and increases in the APG conditions. The changes of convection velocity and skewness factor are caused by influence of large-scale motion through the mechanism called amplitude modulation. The large-scale motion is less active in FPG and more active in APG, therefore in FPG the production of vortices is random (there are no high and low speed regions), while in the APG the large-scale motion drives the production of vortices. Namely, the vortices appear only in the high-speed regions, therefore have convection velocity higher than local mean velocity. The convection velocity affects directly the turbulent sweep and ejection events. The more flow is dominated by large-scale motion the higher values takes both the convection velocity of small-scale structures and sweep events induced by them.

Experimental Analysis of Turbulent Boundary Layer with Adverse Pressure Gradient Corresponding to Turbomachinery Conditions

The paper deals with the experimental analysis of turbulent boundary layer at the flat plate for Reynolds number Re θ ≈3000. The adverse pressure gradient generated by curvature of the upper wall corresponded to the case of pressure distribution in axial compressor. The fully developed turbulence structure was achieved by proper triggering of the boundary layer. The mean and turbulent flow-fields were investigated with the use of hot-wire technique. A substantial effort has been devoted to the precise definition of inlet boundary conditions as well as consistency of measurements obtained with different HWA sensors. The analysis in the paper was concentrated on the problem of scaling of turbulent boundary layer and on the physical background behind scaling laws being compared.

Convection Velocity Variation as a Result of Amplitude Modulation Phenomena

The paper discusses the method of convection velocity estimation in turbulent boundary layer using its relationship with amplitude modulation mechanism. To verify this method the two-point correlation measurements using hot-wire technique was applied in strong adverse pressure gradient flow for two Reynolds numbers. Additionally, streamwise velocity profiles were measured in the same locations. It was shown that the changes in the convection velocity due to Reynolds number and pressure gradient results from amplitude modulation mechanism. The convection velocity in the strong adverse pressure gradient region can be two times higher than the mean velocity in the buffer layer.

Comparison of PIV and Hot-Wire statistics of turbulent boundary layer

The paper shows a cross checking of turbulent boundary layer measurements using large field of view PIV and hot-wire anemometry techniques. The time-resolved PIV method was used for the experiments. The measuring plane was oriented perpendicularly to the wall and parallel to the mean flow. Hot wire measurement has been performed using the special probe with perpendicular hot wire. The HW point measurements were performed in the same place as PIV experiments. The hot-wire probe has the wire length of l+ < 20 in considered range of Reynolds numbers. Various evaluation methods were applied on PIV data. The profiles of statistical characteristics of streamwise velocity components were evaluated from the data. Mean values, standard deviations as well as skewness and kurtosis coefficients were compared for a few values of Reθ. Reynolds number ranges from 1000 to 5500. The result shows that with the increasing Reynolds number the attenuation of fluctuations maximum in PIV measurements occurs with respect to Hot-Wire measurements, however representation of velocity fluctuations using the PIV method is satisfactory. The influence of wall-normal fluctuation component on Hot-Wire near wall peak was also investigated.

An Attempt to Describe Reynolds Stresses of Turbulent Boundary Layer Subjected to Pressure Gradient

The paper is concerned with the issue of scaling of Reynolds stresses and the phenomenon of the outer peak of velocity fluctuations, which appears in adverse pressure gradient conditions. For this purpose, experimental data from favorable and adverse pressure gradient turbulent boundary layers, for Reynolds number varying from \(Re_{\theta }\approx 2300\div 6200\), have been analyzed. At pressure gradient conditions, the self-similarity cannot be obtained using the scale, which is constant across the boundary layer thickness. In this paper, we also propose a modification of the Alfredsson et al. (Eur J Mech B/Fluids 36, 167–175, 2012, [1]) expression, which is dedicated to ZPG flows. The new formulation, utilizing the shape factor H and pressure gradient parameter \(\varLambda \), allows an extension of the validity of Alfredsson et al. proposal for pressure gradient flows.

Analysis of Vortices Generation Process in Turbulent Boundary Subjected to Pressure Gradient

The paper is concerned with the experimental study of bursting process in turbulent boundary layer. For this purpose the novel identification process, developed by Drozdz and Elsner, J Phys: Conf Ser 318(6):062007, 2011, [3] based on VITA technique combined with quadrant analysis was applied. By the detection of four possible combinations of instantaneous gradients of u and v phase-averaged velocity traces this method allows to demonstrate such properties of vortices motion as: swirling direction, ascending or descending direction, the trajectory inclination. The analysis gives an evidence of four types of vortical structures present in the TBL which are responsible for the production of Q-type events, namely prograde and retrograde vortices, with the ascending and descending direction of motion. It was found that detected coherent structures have dominant share of the overall energy of velocity fluctuations.

Study of Reynolds number effect on turbulent boundary layer near the separation

The paper deals with the experimental analysis of strong decelerated turbulent boundary layer developed on a flat plate. The special design of the test section equipped with perforated, movable upper wall allow to generate on the bottom wall turbulent boundary layer, which is at the verge of separation. The objective of the work is to examine the effect of Reynolds number on non-equilibrium boundary layer. A momentum thickness Reynolds number at the inlet to a test section was in the range from 6300 to 10150 what was achieved by varying wind tunnel speed. The emphasis is on the analysis of the streamwise Reynolds stress and mean velocity profiles and on related scaling issues problem. It was found that for the same external conditions defined by the pressure gradient coefficient Cp the effect of Reynolds number close the wall is seen even for such a narrow range of Reynolds number. In particular, the shape factor revealed more flow resistance on separation with increase in Reynolds number. The difference where observed mainly at the beginning of incipient detachment, where there is a drop of turbulence activity near the wall while it reaches a maximum value in the outer region.

Analysis of Development of Vortical Structures in a Turbulent Boundary Layer under Adverse Pressure Gradient Based on VITA Method

The paper concerns experimental investigation of a turbulent boundary layer (TBL) developing on a flat plate under an adverse pressure gradient (APG) for a Re θ ≈ 3000. In particular, the paper deals with the analysis of bursting phenomena in TBL with the use of VITA technique. For interpretation of structure behaviour the shape of conditionally averaged velocity traces recorded by X-wire probe were analysed. It was found that bursting process under the influence of APG is damped near the wall and this phenomenon is emphasized in the outer region of TBL.