| dc.description.abstract | The work presented herein consists of the separation control methods to minimize losses and recover as much velocity head as possible after the divergence in a straight-walled diverging channel in the subcritical flow regime. The study is mainly experimental in nature and extends to different angles of divergence. The methods used are:
(a) Self-Induced Secondary Flow
(b) Self-Rotating Cylinder Hydrofoil Flap (SRCHP)
(c) Trapped vortices at the entry of the channel
(d) Trihedral sills, deflecting plates, and splitter plates
Chapter I
Explains the flow separation phenomenon along the diverging walls and the flow patterns in channels of different divergences. It also outlines the methods of controlling separation in diverging channels.
Chapter II
Describes the experimental setup, details about the instrumentation, and their calibration.
Chapter III
Discusses the creation of Self-Induced Secondary Flow and its effect on velocity head recovery and efficiency of expansion for different angles of divergence.
This method is a suction method, one among the boundary layer control methods. The flow is created by connecting tappings taken from the diverging region to tappings from the approaching flow region by means of bypass tubing. A flow sets in through the tubing from the diverging region to the approaching region, sucking the decelerated fluid particles in the diverging region.
The improvement in efficiency using this method is presented. A theoretical expression for velocity head recovery from momentum considerations has been developed, and the calculated values are compared with the measured values for all angles of divergence. However, this method is more effective at smaller angles of divergence and fails completely if the total diverging angle exceeds 60°.
Chapter IV
Describes separation control using the Self-Rotating Cylinder Hydrofoil Flap (SRCHP). This unique device utilizes the principle of moving boundary as a separation control device.
The cylinder is provided in a streamlined body of profile NACA 23018 and rotates using the existing flow conditions. The construction procedure, location of these hydrofoils in the diverging channel, and their performance with respect to velocity head recovery (with and without rotation) are presented.
Theoretical values of velocity head recovery are compared with experimental values and found to agree well. The drag characteristics of the hydrofoil in decelerated flows have been calculated using the pressure plotting method. Drag characteristics for different angles of attack and deceleration factors, with and without rotation, are presented in Chapter IVb.
Chapter V
Discusses the trapped vortex theory and related experiments to control separation. Results obtained from channels with trapped vortices at the entry and with wooden strips fixed on the sides are presented for 20° divergence. Experiments were confined to 20° because results were not as expected, and the method was not extended to larger angles.
Chapter VI
Describes the performance of other separation control devices in the diverging channel, such as Trihedral sills (three-faced), deflecting plates, and splitter plates. These devices were used only in the fully stalled regime so that the flow is diverted or deflected toward the sides. Splitter plates divide the channel into smaller angles of divergence, providing a relatively smoother pressure gradient.
Calculated values of velocity head recovery from momentum considerations and measured values are compared and found to agree well.
Chapter VII
Presents the three-dimensional flow pattern measured for different angles of divergence with different separation control devices. This study was mainly intended to see whether the secondary flow has any effect on velocity head recovery. Experimental findings and conclusions are given in this chapter.
Chapter VIII
Describes the instruments used in the investigation of the three-dimensional flow field. These instruments were designed and fabricated by the author and calibrated theoretically as well as experimentally. The calibration curves obtained from theory and experiment agree well.
The instruments are Pitot Sphere and Swept Back Edge Impact Probe. However, these probes can measure only the mean behavior of the secondary flow pattern, and no attempt was made to measure the turbulence spectrum of the flow. | |
