Hatsari Mitsudharmadi

Development of boundary-layer flow in the presence of forced wavelength Görtler vortices

Hot-wire measurements in the boundary layer developing on a concave surface of 2.0 m radius of curvature in the presence of forced wavelength Gortler vortices have been conducted for a free-stream velocity of 3.0 m/s. The wavelengths of vortices were preset by vertical perturbation wires of 0.2 mm diameter located 10 mm upstream of the concave surface leading edge. The velocity contours in the cross-sectional planes at several streamwise locations show the growth and breakdown of the vortices that are similar to those found in the transitional flow field. It shows the occurrence of the second instability mode that is indicated by the formation of small horseshoe eddies generated between the two neighboring vortices traveling in the streamwise direction to form mushroom-like structures as a consequence of the nonlinear growth of the Gortler vortices. The breakdown of these structures before the boundary-layer flow becomes turbulent is also shown to qualitatively predict the start of the transition in the f...

Visualizing Görtler Vortices

The development of Görtler vortices with pre-set wavelength of 15 mm has been visualized in the boundary-layer on a concave surface of 2.0 m radius of curvature at a free-stream velocity of 3.0 m/s. The wavelength of vortices was pre-set by vertical wires of 0.2 mm diameter located 10 mm upstream of the concave surface leading edge. The velocity contours in the cross-sectional planes at several streamwise locations show the growth and breakdown of the vortices. Three different regions can be identified based on different growth rate of the vortices. The occurrence of a secondary instability mode is indicated by the formation of a small horseshoe eddies generated between the two neighboring vortices traveling streamwise, to form mushroom-like structures as a consequence of the non-linear growth of the Görtler vortices.

Effect of Rounded Edged Dimple Arrays on the Boundary Layer Development

The effect of a turbulent boundary layer subjected to a series of rounded edged shallow dimple arrays with dimple depth ratios, d/D of 4%, 8% and 12% were experimentally studied. Measurements show the existence of a higher flow speed region at the center of each dimple. The spanwise distribution of the mean wall shear stress immediately downstream of the centers of the last row of dimples does not vary with dimple depth, and is about 45% over that without the dimple array. Turbulence measurements and surface flow visualization shows that the flow over the shallowest dimple differs from the deeper dimples. Flow separation observed with the deeper rounded edged dimples produce similar flow structures as those from sharp edged dimples reported in the literature. However flow separation is not observed when d/D=4% but instead two other higher speed regions either side accompany the central flow. The effects of the dimples are rapidly suppressed by the flat surfaces between of the dimples, and the flow rapidly reverts back to an unmanipulated flat boundary layer flow in these areas.

Secondary instability in forced wavelength Görtler vortices

From an experimental study on the onset of secondary instability in forced wavelength Gortler vortices, it is found that the breakdown process of the Gortler vortices is due to the development of varicose and sinuous mode instabilities. The development of the varicose mode is characterized by the formation of horseshoe vortices that evolve downstream to form mushroom-like structures. This phenomenon is then followed by meandering of the vortices as an indication of the onset of sinuous mode instability, prior to turbulence. The spectrum analysis applied to the fluctuating velocity component shows the occurrence of peak frequency of about 150Hz, which is attributed to the fundamental secondary instability mode with its wavelength comparable to the spanwise wavelength of the primary Gortler vortices, at the location where the mushroom-like structures are clearly depicted in the mean velocity contours on the y-z plane. This confirms that the secondary instability is of the varicose type at the onset that is ...

Development of most amplified wavelength Görtler vortices

The development of most amplified wavelength Gortler vortices is studied by means of varying the spanwise spacing of thin vertical wires located upstream of the leading edge of a concave surface. The free-stream velocity is set so as to provide the value of the dimensionless parameter of that for the most amplified vortex wavelength. The resulting uniform vortex wavelengths were determined by the wire spacings and they were preserved downstream prior to turbulence. The spectrum study of the fluctuating velocity component was able to detect the fundamental frequency of the secondary instability mode with the streamwise wavelengths comparable to the wire spacing, which confirm that the wavelength of the vortices observed is the most amplified one. The intermittency study of the boundary layer flow in the presence of the most amplified wavelength Gortler vortices of 15.0 mm using Turbulent Energy Recognition Algorithm method shows the transition onset in the upwash regions, which coincides with the onset of ...

Splitting and merging of Görtler vortices

The splitting and merging of Gortler vortices are experimentally studied by varying the spacings of vertical wires located 10mm upstream of a concave surface leading edge of 2.0m radius of curvature subjected to a free-stream velocity of 3.0m∕s. The splitting and merging as the result of the linear instability of the vortices with respect to spanwise perturbation (Eckhaus instability), occurred when the wire spacing was set to respectively twice and half of the dominant or most amplified wavelength of Gortler vortices. These show the susceptibility of Gortler vortices to wavelengths greater and smaller than the most amplified wavelength of the vortices. The spectral study shows that the values of the dimensionless frequency parameter for the wire spacings of 7.5, 15.0, and 30.0mm are nearly constant (of about 0.5) for the streamwise locations where the mushroom-like structures dominate the flow. It is also found that the dimensionless wavelength parameter is more sensitive than the Reynolds number of the ...

Visualization of pre-set vortices in boundary layer flow over wavy surface in rectangular channel

Smoke-wire flow visualization is used to study the development of pre-set counter-rotating streamwise vortices in boundary layer flow over a wavy surface in a rectangular channel. The formation of the vortices is indicated by the vortical structures on the cross-sectional plane normal to the wavy surface. To obtain uniform spanwise vortex wavelength which will result in uniform vortex size, two types of spanwise disturbances were used: a series of perturbation wires placed prior and normal to the leading edge of the wavy surface, and a jagged pattern in the form of uniform triangles cut at the leading edge. These perturbation wires and jagged pattern induce low-velocity streaks that result in the formation of counter-rotating streamwise vortices that evolve downstream to form the mushroom-like structures on the cross-sectional plane of the flow. The evolution of the most amplified disturbances can be attributed to the formation of these mushroom-like structures. It is also shown that the size of the mushroom-like structures depends on the channel entrance geometry, Reynolds number, and the channel gap.Graphical Abstract

Concave Surface Boundary Layer Flows in the Presence of Streamwise Vortices

Concave surface boundary-layer flows are subjected to centrifugal instability which results in the formation of streamwise counter-rotating vortices. Such boundary layer flows have been experimentally investigated on concave surfaces of 1 m and 2 m radius of curvature. In the experiments, to obtain uniform vortex wavelengths, thin perturbation wires placed upstream and perpendicular to the concave surface leading edge, were used to pre-set the wavelengths. Velocity contours were obtained from hot-wire anemometer velocity measurements. The most amplified vortex wavelengths can be pre-set by the spanwise spacing of the thin wires and the free-stream velocity. The velocity contours on the cross-sectional planes at several streamwise locations show the growth and breakdown of the vortices. Three different vortex growth regions can be identified. The occurrence of a secondary instability mode is also shown as mushroom-like structures as a consequence of the non-linear growth of the streamwise vortices. Wall shear stress measurements on concave surface of 1 m radius of curvature reveal that the spanwise-averaged wall shear stress increases well beyond the flat plate boundary layer values. By pre-setting much larger or much smaller vortex wavelength than the most amplified one, the splitting or merging of the streamwise vortices will respectively occur.

Visualization of Counter-rotating Streamwise Vortices in a Rectangular Channel with One-sided Wavy Surface

Counter-rotating streamwise vortices in a rectangular cross-section channel with a wavy surface on one side were visualized by smoke-wire flow visualization technique. The use of six equilateral triangle patterns cut at the leading edge of the wavy surface results in relatively uniform spanwise wavelengths of these vortices. Visualization results show that these vortex structures remained visible until the second peak of surface waviness in the channel for Re = 3350, while for Re = 5300 more engulfment occurs in the spanwise structures of the vortices at the first peak of the surface waviness. The more engulfment could increase the mixing which results in early breakdown of the streamwise vortices prior to turbulence. It is also revealed that the evolution of these counter-rotating streamwise vortices is confined within the boundary layer. It causes the spanwise variation of the boundary layer thickness indicated by the spanwise wavy pattern at the edge of the boundary layer.

On the Development of Concave Surface Boundary Layer Flows

The development of concave surface boundary‐layer flows is characterized by the formation of streamwise counterrotating Gortler vortices and has been experimentally investigated for concave surfaces of 1 m and 2 m radius of curvature. For this study, the wavelengths of the vortices were pre‐set or “forced” by thin perturbation wires placed upstream and perpendicular to the concave surface leading edge. This method was used to obtain uniform vortex wavelengths. Velocity contours were obtained from velocity measurements using a single hot‐wire anemometer probe. The most amplified wavelengths of the vortices can be pre‐set by the spanwise spacing of the thin wires and free‐stream velocity. The velocity contours on the cross‐sectional planes at several streamwise locations show the growth and breakdown of the vortices. Three different regions can be identified based on the growth rate of the vortices. The occurrence of a secondary instability mode is also shown in the form of mushroom‐like structures as a con...