Sohrab S. Sattarzadeh

Experimental investigation on the steady and unsteady disturbances in a flat plate boundary layer

Recent experiments have shown that miniature vortex generators (MVGs) are coveted devices to stabilize unsteady disturbances in flat plate boundary layers and to delay the onset of turbulence by modulating the base flow in the spanwise direction. The spanwise modulation is a result from the non-modal transient growth of steady and spanwise periodic streamwise vortices being generated by the MVGs. The present experimental investigation aims at studying the transient growth of non-modal disturbances induced by a spanwise periodic array of MVGs and its stabilizing effect on non-linear unsteady disturbances in the boundary layer originating from planar Tollmien-Schlichting (TS) waves. Measurements consist of cross-stream planes at different downstream locations in the boundary layer and a spatio-temporal analysis of different modes of the disturbances is carried out. In the streaky boundary layer generated by the MVGs the fundamental spanwise mode, with the same wavelength as the MVG pairs in the array, and its first harmonic, both undergo transient growth whereas the higher harmonics decay immediately downstream of the array. In the unstable region formed in the wake of the MVG blades, i.e., just downstream of the array, a wide range of spanwise modes contributes to an initial growth in the energy of unsteady disturbances. Similar behavior is observed upstream of branch II position of the neutral stability curve where the unsteady disturbances undergo a second energy growth in the unstable region. It is shown that the spatial gradients of the base flow in the wall-normal and spanwise directions are contributing to the amplification and attenuation of the TS wave disturbances, respectively, in the streaky boundary layer.

Hot-Wire Calibration at Low Velocities: Revisiting the Vortex Shedding Method

The necessity to calibrate hot-wire probes against a known velocity causes problems at low velocities, due to the inherent inaccuracy of pressure transducers at low differential pressures. The vortex shedding calibration method is in this respect a recommended technique to obtain calibration data at low velocities, due to its simplicity and accuracy. However, it has mainly been applied in a low and narrow Reynolds number range known as the laminar vortex shedding regime. Here, on the other hand, we propose to utilize the irregular vortex shedding regime and show where the probe needs to be placed with respect to the cylinder in order to obtain unambiguous calibration data.

Advanced Fluid Research On Drag reduction In Turbulence Experiments –AFRODITE–

A hot topic in todays debate on global warming is drag reduction in aeronautics. The most bene cial concept for drag reduction is to maintain the major portion of the airfoil laminar. Estimations ...

Revisiting the Near-Wall Scaling of the Streamwise Variance in Turbulent Pipe Flows

Apparent contradictory results regarding the Reynolds number scaling of the near-wall peak of the variance distribution in turbulent pipe flows are discussed. Inconsistencies in the conclusions from the Princeton SuperPipe published between 2010-2012 are highlighted and new experimental evidence in the Karman number range 500-2500 is supplemented. The new results support the view that the innerscaled peak amplitude increases with Reynolds number as for channel and turbulent boundary layer flows, and in agreement with trends observed in recent direct numerical simulations and other experiments.

Mastering nonlinear flow dynamics for laminar flow control

A laminar flow control technique based on spanwise mean velocity gradients (SVGs) has recently proven successful in delaying transition in boundary layers. Here we take advantage of a well-known nonlinear effect, namely, the interaction of two oblique waves at high amplitude, to produce spanwise mean velocity variations. Against common belief we are able to fully master the first stage of this nonlinear interaction to generate steady and stable streamwise streaks, which in turn trigger the SVG method. Our experimental results show that the region of laminar flow can be extended by up to 230%.

Transition to Turbulence Delay Using Miniature Vortex Generators – AFRODITE –

A laminar boundary layer has a relatively low skin-friction drag coefficient (c f ) with respect to a turbulent one, and for increasing Reynolds number the difference in c f rapidly increases, and the difference can easily amount to an order of magnitude in many industrial applications. This explains why there is a tremendous interest in being able to delay transition to turbulence, particularly by means of a passive mechanism, which has the advantage of accomplishing the control without adding any extra energy into the system. Moreover, a passive, control does not have to rely on typically complicated sensitive electronics in sensor-actuator systems. Within the AFRODITE project [3] we now present the first experimental results where we are able to show that miniature vortex generators (MVGs) are really coveted devices in obtaining transition delay.