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Chestermiller said:It seems to me that there is going to be substantial drag along the length of the bar as well as at the leading face. The length is more than 10x the cross sectional dimensions. What is the free stream velocity of the water flow going to be along the bar?
Chet
http://www.aerospaceweb.org/question/aerodynamics/q0231.shtmlvenkiee said:Actually it is a sacrificial anode welded on a tubular underwater for an offshore structure. I's appreciate if can advise how to calculate its drag coefficient CD value
It's not going to matter much to the CD that the cross section of the anode is a rectangle, a trapezoid, a circle, or whatever. You have a flat surface which is perpendicular to the flow. The CD is going to be in the range 0.82 - 1.28, with the lower number being for flow end onto a long cylinder and the higher number being for flow directly onto a flat plate:venkiee said:Actually it is a sacrificial anode welded on a tubular underwater for an offshore structure. I's appreciate if can advise how to calculate its drag coefficient CD value
It's not clear how big a structure these anodes are protecting, nor how many anodes there are attached to the structure in this manner. If the flow is along the length of the anode, that suggests it's also along the length of the tubular, which should generate less resistance than if the flow was normal to the side of the tubular.venkiee said:This is how my attachment looks. Tube has its own CD factor. But when an anode is attached to it, for increased CD on the tubular, the attachment has to be considered for analysis purpose to calculate wave drag on main tubular. My question is the anode part. There are no clear guidelines how to calculate CD CM for such attachment as individual...
Appreciate detailed explanation and method.
SteamKing said:It's not clear how big a structure these anodes are protecting, nor how many anodes there are attached to the structure in this manner. If the flow is along the length of the anode, that suggests it's also along the length of the tubular, which should generate less resistance than if the flow was normal to the side of the tubular.
While providing the CD CM values for members, if they are tubular, it is straight forward. If there is an attachment to it (like anodes) the CD CM need to be modified to counter the added mass and drag forces.SteamKing said:Thanks for the info.
Are you trying to evaluate the wave forces on the jacket structure after it has been installed?
It's not a pipeline.JBA said:Second question, is this a pipeline lying on the seabed or is something elevated above the seabed?
SteamKing said:It's not a pipeline.
From the pitchers the OP posted, it's a jacket erected offshore, like to support an oil production platform. The anodes are affixed to the tubular structural members to mitigate corrosion from seawater. It appears the OP is trying to evaluate wave loads imposed on the fixed structure, as if to perform a structural analysis.
venkiee said:While providing the CD CM values for members, if they are tubular, it is straight forward. If there is an attachment to it (like anodes) the CD CM need to be modified to counter the added mass and drag forces.
Hence, when the jacket is in installed / operating condition in sea water, this situation arises. There are no clear cut indications how to arrive a CD CM for attached anode. That's why I am asking...
It should be like what is shown in figure above for a trapezoidal bar
JBA said:From the statements with the pictures he does not state that his application is a jacket structure, only that the anode arrays are similar to what is shown on the structures tube members. No offence intended, only clarification. Hopefully Venkiee will resolve this question.
SteamKing said:I think the application is indeed on a jacket. Pipelines can be buried under the seabed, and no one needs to calculate wave forces acting on a pipeline, which they are required to do for fixed platform installations offshore, especially if the structure is being classed by one of the marine classification societies (ABS, DNV-GL, etc.)
I mentioned clearly that It's a fixed Jacket Structure.JBA said:From the statements with the pictures he does not state that his application is a jacket structure, only that the anode arrays are similar to what is shown on the structures tube members. No offence intended, only clarification. Hopefully Venkiee will resolve this question.
Seems like there's a discussion in the linked paper of the drag due to the anodes and how it affects the overall drag of the tubular to which it is attached.venkiee said:I mentioned clearly that It's a fixed Jacket Structure.
Sacrificial anodes are attached to the Jacket to avoid corrosion of steel under water.
But when the anodes are attached to the tubular members, the members attract additional drag and inertia forces due to shape change. for this kind of shape in an open channel flow, there is no drag coefficient defined in any literature.
Hence I asked if there can be any assistance to do a calculation to ascertain this CD value for a trapezoidal bar attachment.
JBA said:The fact that the design and successful application of conventional jacket structures in some of the most severe oceanic wave regions around the world for many years and multiple orientations of the jacket members relative to wave orientations simply caused me to wonder why the drag of an anode in one particular flow orientation was one of special interest.
I think this is why the engineers at Petronas decided to do model testing. A purely analytical solution appears to be unlikely; therefore, the gap in the theory must be filled by model testing.venkiee said:As the anodes contribute around 5% to 8% of jacket weight, they play a significant role in increase of wave loads and the pile head reactions. In Well head jackets (usually light) the gravity loads contribute only 30% while environmental loads occupy rest 70%.
The increase in CD CM has a major role in pile head forces up to 10% in general.. If you don't add CD CM for that anodes.. you can notice the difference
This is a big topic of discussion... can't be explained in few words...
The drag coefficient of a trapezoidal bar is a dimensionless quantity that represents the resistance to motion caused by the shape of the bar. It is denoted by the symbol "Cd" and is dependent on various factors such as the shape, size, and speed of the bar.
The drag coefficient of a trapezoidal bar can be calculated using the formula: Cd = 2Fd/ρAV², where Fd is the drag force, ρ is the density of the fluid, A is the reference area, and V is the velocity of the bar.
The reference area used for calculating the drag coefficient of a trapezoidal bar is the projected area of the bar when viewed from the direction of motion. It is usually the cross-sectional area of the bar at its widest point.
The shape of a trapezoidal bar plays a significant role in determining its drag coefficient. The more streamlined the shape, the lower the drag coefficient will be. A more tapered or pointed shape will have a lower drag coefficient compared to a square or blunt shape.
Apart from the shape and size of the bar, the drag coefficient can also be affected by the fluid properties such as density and viscosity, as well as the speed and angle of the bar's motion. The surface roughness and the presence of any roughness elements on the bar can also influence the drag coefficient.