Bu çalışmada, deneysel olarak geri basamak akışı ve akım ayrılması incelenmiştir. İlgili literatüre tam ve yeni bir seri basınç dağılımı, kayma gerilmesi, hız ve türbülans verisi sunulması hedeflenmiştir. Geri basamak akışı sıklıkla sayısal yöntemler ile hazırlanan bilgisayar kodları ve türbülans modelleri için bir test akışı olarak kullanılmaktadır. Bu çalışma, sayısal çalışmalar için güvenilir deneysel veri sağlamayı da amaçlamaktadır. Deneylerde ortak genel boyutlara sahip aynı aileden farklı geometrilerde basamak modelleri kullanılmıştır. Geri basamak akışının incelenmesinde İTÜ Trisonik Aerodinamik Laboratuarına ait Eiffell tipi açık devreli sesaltı bir hava tüneli kullanılmıştır. Basamak yüksekliği 20 mm olmak üzere 5 değişik basamak geometrisi üzerinde deneyler gerçekleştirilmiştir. Deneyler sırasında hava tünelinin serbest akım hızı 20 m/s, serbest akım türbülans değeri ise % 0.5’dir. Basamak yüksekliği gözönüne alınarak hesaplanan Reynolds sayısı 2.74x104’tür. Basamak modellerinin hepsinde açıklık oranı 40’tır. Akımın hız ve türbülans değerleri, ayrıca yüzeydeki kayma gerilmeleri bir sıcak tel anemometresi kullanılarak ölçülmüştür. Basamak öncesi ve sonrası basınç dağılımları yüzey üzerinde çeşitli noktalardaki statik basınç değerlerinin zaman ortalamaları alınarak belirlenmiştir. Yüzeydeki akım çizgileri ve akım yönünü görünür kılabilmek maksadıyla yüzey yağ-film tekniği kullanılmıştır. Elde edilen sonuçlar literatürdeki diğer sonuçlarla uyum sağlamaktadır. Akıma dik yöndeki yüzeyi çeyrek daire formunda bir eğime sahip olan basamak modelinin diğer modellere göre daha kısa bir yeniden yapışma uzunluğuna sahip olduğu belirlenmiştir.

Flow separation is one of the complicated aspects of viscous flow. It is a very important phenomena not only for science, but also for practical applications. Separating flow over two dimensional backward facing steps is the simplest class of separated flows because the separation point is fixed and the flow leaves the boundary at zero angle of separation. The separation line is straight and fixed at the edge of the step, and there is only one separated zone instead of two, as seen in the flow over a fence or an obstacle. In addition, the streamlines are nearly parallel to the wall at the separation point, so significant upstream influence occurs only downstream of separation. Because of these features of the backward facing step flow most of the research on separated flows has been done on it. Although backward facing step flow offers one of the least complex separating and reattaching flows, the flowfield is still very complex. There are several parameters influencing the physical properties of the flowfield. Investigations conducted by a number of independent researchers using different techniques have numerous variations of the results. Although all of these researches are in good quality and they separately serve to different purposes, much work is still needed in this subject. In the present work the backward facing step flow was chosen to investigate flow separation experimentally. The primary purpose is to add a complete and new set of surface pressure, shear stress, mean velocity and turbulence data to the relating literature. To achieve this purpose different step geometries with common general dimensions were used. The step configurations in the available literature are generally sudden expansion type and channel type geometries with tunnel walls. To prepare a different data, a configuration with free upper boundary and high aspect ratio (step width/step height) was used. The backward facing step flow is also used often as a test case for CFD codes and turbulence models. Present study is aimed to provide reliable experimental data for numerical studies. The main purpose is expanding the physical understanding of backward facing step flow along with flow separation. In order to investigate backward facing step flow an Eiffell type open circuit subsonic wind tunnel of ITU Trisonic Aerodynamics Laboratory was used. With the same height of 20 mm 5 different step geometries were employed in a 20 m/s freestream velocity of a freestream turbulence intensity of 0.5%. Reynolds number based on the step height was 2.74x104. Step geometries with different bases in addition to basic step were 2 slanted bases of 45 degrees one in the upstream direction and the other in downstream direction and 2 circular bases one a half circle section and the other a quarter circle section. Aspect ratio of the step models was 40. A strip of sand paper was used 24 step height upstream of the step to ensure the turbulent flow. Although separating and reattaching flows have generally an unsteady nature, present study was interested in time-averaged values of flow properties. Vortex shedding and instability in the free-shear layer were not treated. Mean velocity and turbulence values of the flow and shear stress values on the wall of the steps were measured by utilizing a constant temperature hot wire anemometer. For the mean velocity and turbulence experiments the locations of measurements were between 3 step height upstream and 20 step height downstream of the step. Pressure distribution upstream and downstream of the step surface was obtained by measuring the time-averaged static pressure at various points. The surface oil-film technique was used to visualize the surface streamline or flow direction on the surface by coating the surface of the step model with oil film. All the measurements mentioned above were performed on the centerline of the step models in the direction of the freestream flow, where flow properties were minimally effected by the three dimensionality of the flow. A complete and new set of data with additional support of flow visualizations were presented. Resulting data represent logical behavior for the model geometries. Basic step case has the results agreeing with the available literature which have similar Reynolds number values and flow characteristics. Curved step cases have smaller recirculation region, while the positive and negative slanted step cases has no significant difference than the basic step case.